The goal of this research is to elucidate fundamental mechanisms of radiation damage to DNA by radiations of varying linear energy transfer (LET). Our comprehensive model for DNA radiation damage that describes events from the initial formation of DNA ion radicals and excited states, to hole and electron transfer, to sugar radical formation and finally to molecular products will be tested at each step to clarify the fundamental processes resulting in DNA radiation damage. These studies, which are performed under conditions that emphasize the direct effect of radiation, will employ magnetic resonance spectroscopies, density functional theory and product analysis techniques as well as gamma and cyclotron heavy ion beam irradiations. There are three aims: The first aim will address several of the major unanswered questions in DNA radiation damage induced by holes. This aim will employ specifically C-8 deuterium labeled defined sequence oligos to exploit a recent breakthrough in our laboratory that allows us to distinguish a hole (cation radical) at a C-8 deuterium labeled purine base (guanine or adenine) from an unlabeled site. We have also found that the C-8 labeling allows the distinction of the guanine and adenine cation radicals from their deprotonated forms. With these developments we will find: a. the base sequence dependence of hole localization, b. the protonation states of guanine and adenine cation radicals at specific sites in dsDNA, c. the extent of base-to- base versus base-to-sugar transfer on hole excitation. Our second aim will identify radicals formed and track structure as a function of LET in ion beam irradiated DNA. We will identify radicals via ESR spectroscopy and ascertain their spatial distribution and clustering as a function of the LET of the radiation along the radiation track. Especially important will be a study of the LET dependence of recently discovered prompt strand break radicals that result from cleavage of the sugar phosphate backbone. The nature of the radical formation and clustering in the track core is pertinent to understanding the formation of the most important lesion in DNA the unrepairable multiply damaged site. Our final aim will employ theoretical calculations to further test and confirm molecular mechanisms proposed in the above studies. Especially significant will be treatment by TD- DFT theory of excited states of base ion radicals which are now implicated in DNA strand breaks and become more significant as the LET of the radiation increases. We believe this effort will allow us to establish new insights into fundamental radiation processes important for biomedical research. PUBLIC HEALTH RELEVANCE: The goal of this research is to develop a comprehensive model of DNA radiation damage by elucidating fundamental mechanisms of damage to DNA by radiations of varying linear energy transfer (LET). Our model for DNA radiation damage that describes events from the initial formation of DNA ion radicals and excited states, to hole and electron transfer, to sugar radical formation and finally to molecular products will be tested at each step to illuminate the fundamental processes resulting in DNA radiation damage. These studies, which are performed under conditions that emphasize the direct effect of radiation, will employ gamma and cyclotron heavy ion beam irradiations, magnetic resonance spectroscopies, density functional theory and product analysis techniques and will address major unanswered questions in DNA radiation damage important to biomedical research.